The present invention relates in general to air-pressure forming of sheets of thermoplastic resin and particularly to a method of and an apparatus for carrying out air-pressure forming under an air pressure controlled at a substantially constant level.
There are known a variety of methods of producing from a sheet of thermoplastic resin material products of various shapes such as containers for ices and beverages, containers for food products, etc., such as vacuum forming, air-pressure forming, combination process thereof, mechanical molding, etc. However, these known methods and the apparatuses therefor are inherently accompanied by such technical and economical problems as follows.
1. Considerably costly equipment such as a vacuum pump, its accessory devices, a compression pump, its accessory devices, and the like are required. PA1 2. These vacuum pumps and compression pumps are liable to produce considerably noises that cause a serious impairment of the work environment. PA1 3. Such electrically-operated devices as solenoid valve, relays, timers, and many others, are essentially required and are likely to cause trouble particularly when a molding operation cycle is cut short. PA1 4. Pipings from such vacuum and compression pumps to the forming station become longer, thus resulting in substantial power losses. PA1 5. When compressed air required in performing a forming operation flows through the piping, time losses are incurred. Consequently, it becomes difficult to continuously perform forming operations at a high production rate and with a large quantity of products. PA1 1. Variation in forming pressure occurs with changes of the volume of the material to be formed. PA1 2. A forming pressure variation occurs between the initial forming stage and the middle and end stages where the formed products are subjected to cooling under pressure. PA1 3. Pulsation in forming pressure occurs as a result of the reciprocating motion of the piston for compression action. PA1 4. Leakage of compressed air occurs when there is a difference in the thickness of a material to be formed, therefore requiring make-up of compressed air.
In this respect, an air-pressure forming apparatus as a countermeasure to such problems has been proposed and disclosed in Japanese Patent Publication No. 14492/1963. This apparatus comprises a cylinder having a piston therein arranged slidably toward and away from a master disc set on the base of the apparatus, thereby applying a pressure upon the back side of a sheet of material to be formed with lowering or compressing motion of the piston in the cylinder, thus forming a disc surface under pressure. However, when this apparatus is used in forming operations, it is impossible to apply a pressure to the material sufficient to effect a proper forming during the initial stage of compression by the piston due to a relatively slow initial pressure rise, whereby the pressure rising within the cylinder becomes highest when the piston comes down to its lowest position or end of compression stroke. Consequently, there is the possibility of the formed products turning out to be of inferior quality.
In order to overcome this problem, a pressure forming method has been proposed and disclosed in the Japanese Patent Laid-Open No. 44074/1974, in which the following problems cannot be avoided.
In this respect, it would be advantageous if an improved method and apparatus were to be provided to overcome the above described problems. This invention is essentially intended to provide a useful method of forming a sheet of thermoplastic resin and an apparatus therefor, whereby a hot forming method and an apparatus therefor are attained which can overcome all of the problems stated above.